Check for javascript results string from PHP echo not working

Question

I'm a beginner with JavaScript, what I need to do is to show a div with an alert in case of a certain string in the results data after a JavaScript function has been executed.

Here's my code (I'm working with Parse PHP SDK):

comments.php

echo '
      <h5>'.$topic.'</h5>

      <div style="display:none;" id="anAlert" class="alert alert-danger"> 
      <button type="button" class="close" data-dismiss="alert">×</button> <strong>Ouch!</strong>
                You already upvoted this Topic!
            </div>
            <br>
            ';

Javascript code into comments.php:

// UPVOTE A TOPIC -------------------                                      
function upvoteTopic() {
    var topicID = "<?php echo $topicID ?>";
    var votesNr = Number("<?php echo $votes ?>"); 

    $.ajax({
        url:"vote-topic.php?isUpvoted=true&topicID=" + topicID, // Call vote-topic.php 
        type: "GET", 

        success:function(data) {
            var results = data
            console.debug(results);

            if (results !== 'You already upvoted this Topic') {

                // Show updated votes
                document.getElementById('votesTxt').innerHTML = votesNr;

                // Change buttons style
                $( "#upvoteTopicButt" ).removeClass( "btn-default" ).addClass( "btn-primary" );
                $( "#downvoteTopicButt" ).removeClass( "btn-primary" ).addClass( "btn-default" );

            } else {
                document.getElementById("anAlert").style.display = 'block';
            }

    }});
}

vote-topic.php

// UPVOTE ------------------------------------------
if ($isUpvoted == "true") {

    // CANNOT VOTE YOUR OWN TOPIC
    if($userPointer->getObjectId() == $currUser->getObjectId()) {
        echo 'You cannot vote your own Topic!';

    // VOTE TOPIC
    } else {

        // Get votes array
        $upvotedBy = $tObj->get('upvotedBy');
        $downvotedBy = $tObj->get('downvotedBy');

        // You already upvoted this Topic
        if (in_array($userID, $upvotedBy)) {

            echo 'You already upvoted this Topic'; // HERE'S THE LINE I NEED TO COMPARE IN THE IF STATEMENT OF MY JAVASCRIPT FUNCTION!

        // UPVOTE TOPIC
        } else {
            array_push( $upvotedBy, $currUser->getObjectId() );
            $downvotedBy = array_diff( $downvotedBy, array($currUser->getObjectId() ) );

            // Increment votes
            $tObj->increment("votes", 1);
            $tObj->setArray('upvotedBy', $upvotedBy);
            $tObj->setArray('downvotedBy', $downvotedBy);
            $tObj->save();

            // echo 'You upvoted this Topic!';
        }
    }

So, when I click the UPVOTE button, my Javascript function successfully calls the vote-topic.php script and performs the saving in there, it changes the style of the upvoteTopicButt and downvoteTopicButt buttons, but it doesn't display my anAlert div.

I've tried to launch an alert of results, and it actually shows this line:

You already upvoted this Topic

So that's why I've placed this IF statement into my Javascript function:

    if (results !== 'You already upvoted this Topic') {
      ...

    } else {
        document.getElementById("anAlert").style.display = 'block';
   }

In theory, when the results message is == You already upvoted this Topic, my function should show my anAlert div. But it doesn't.

What am I doing wrong?

Answer 1

Compare vote-topic.php response as a result may be tricky if you have undesired char before or after your string. You can return a JSON or working with HTTP response code to "throw" an error.

For example with different error code:

Change echo 'You already upvoted this Topic'; by http_response_code(418);

Then, in your javascript file, add an error function like:

$.ajax({
    url:"vote-topic.php?isUpvoted=true&topicID=" + topicID, // Call vote-topic.php 
    type: "GET", 

    success:function(data) {
        var results = data
        console.debug(results);

        // Show updated votes
        document.getElementById('votesTxt').innerHTML = votesNr;

        // Change buttons style
        $( "#upvoteTopicButt" ).removeClass( "btn-default" ).addClass( "btn-primary" );
        $( "#downvoteTopicButt" ).removeClass( "btn-primary" ).addClass( "btn-default" );
    },
    error: function () {
        document.getElementById("anAlert").style.display = 'block';
    }
});

See jquery ajax doc